1. Field of the Invention
The present invention is directed to the field of welding and more particularly to a welding system utilizing a ground integrity monitor and to a method for monitoring the resistance of a ground conductor.
2. Description of the Prior Art
Hand-welding guns are welding guns that are manually brought to their position on the workpiece to be welded. They are usually fastened to a balancer, i.e., a balancing mechanism, which bears the weight of the welding gun and facilitates the manual alignment of the welding gun.
There are essentially two different types of hand-operated welding guns, namely a first type without an integrated welding transformer and a second type having an integrated transformer. In the first type, the transformer itself is fastened to a rod of the balancer and does not follow all the movements of the welding gun. The disadvantage of these welding guns is that they require long cables for the secondary power supply. The secondary power exhibits very high amperage, for example, 10,000 A. The secondary power supply therefore requires very thick conductors, and over long distances, generates considerable loss because of the conductor resistance.
For this reason, transformer guns in which a welding gun including an integrated transformer is fastened to the balancer are preferred as hand-welding guns. Though a transformer gun may, because of the transformer, weigh more than a welding gun without a transformer, it generates insignificant losses because of the short secondary high-amperage power cables.
A disadvantage of a transformer gun is that a medium voltage of 500 V, for instance, which may be dangerous to the operating personnel, is supplied to its transformer. For this reason, increased safety demands are set for transformer guns.
Such types of welding guns are monitored through ground fault current interrupters. A ground fault current interrupter monitors the fault current to ground. Ground fault current interrupters, or GFCIs, are adequately known from the related art. In welding equipment, normal GFCIs disconnect the power supply within a very short time, e.g., 30 ms, in the event of fault currents between 10 and 30 mA.
In the U.S. in particular, an additional safety circuit is required. This is achieved through safety devices in the primary power supply, which switch off the electric power supply in the event of a discharge of the primary power to ground. For this safety circuit, it is necessary for the ground conductor of the casing of the welding guns to be operative. In order to ensure the reliable functioning of this additional safety device, the integrity of the ground conductor is monitored. This testing device is normally called a “ground integrity monitor” and will be referred to as such within the scope of this application.
The ground integrity monitor conducts a testing current, e.g., 20 mA, through the ground conductor between the casing and the control box for the welding power supply. The voltage generated through this testing current is measured between the start and end of the ground conductor and the resistance is deduced therefrom. If the resistance determined exceeds an acceptable value of, for instance, 1 Ω, the welding power supply is cut off. The power is switched off very quickly, e.g., within less than one second.
During the welding, as a result of interference effects, the welding current is partially conducted from one electrode to the other not through the workpieces, but through the ground conductor. The workpieces themselves are also grounded, so that current may flow from the first electrode through the workpiece to ground, and subsequently, through the ground conductor to the welding gun casing and back to the second electrode. This welding current flowing through the ground conductor may result in malfunctions in the ground integrity monitor. In the past, when measuring the resistance of the ground conductor for the casing, there were attempts to filter out the welding current flowing through the ground conductor. This was not always totally successful, however, resulting in wrong welding power shut-downs occurring when the ground conductors were intact.
It is desirable to reduce the danger of shut-downs through the ground integrity monitor during the welding operation despite an intact ground conductor.